Manhole assemblies have been found to typically experience significant interior corrosion and deterioration even in cases where high acidic effluents that are known to be harmful to sewers and sewer treatment are prohibited from entering the sewers before first being dissipated or neutralized. Nevertheless, hydrogen sulfide which is inherent in sewage, is developed due to the presence of sulfur compounds, such as sulfate, sulfite, or other inorganic or organic sulfur. The above-mentioned group of compounds are reduced to sulfide by sulfate-reducing bacteria normally found in the effluent. The generation of hydrogen sulfide is accelerated in the presence of high temperature and low flow rates. The useful life of concrete is determined by dividing the available effective thickness of the concrete by the corrosion rate. The corrosion rate can be calculated when all factors are known. The effective thickness of the concrete is the amount covering the steel reinforcement typically embedded within the manhole assembly.
Coatings have been applied to manhole interiors but have been found to have a poor track record. For example, although coal, tar, or epoxy provides effective protection against hydrogen sulfide, such coatings have provided poor field performance due to application difficulties.
As a result, linings of plastic material, such as polyvinyl chloride (PVC), provide the best performance for interior corrosion protection against hydrogen sulfide. Such plastic linings are further compatible with plastic pipe now being used extensively in sanitary systems. To date, however, it is extremely difficult to fabricate interior linings and integrate such interior linings into vertical structures and particularly manhole assemblies. Flexible type linings are presently used in pipes covering the upper 270.degree. of the pipe interior. This portion is attacked by the H.sub.2 S generated from sewage. This flexible material is not easily used on manholes which require 360.degree. protection for the manhole interior.
U.S. Pat. No. 4,751,799, issued June 21, 1988 and assigned to the assignee of the present invention, discloses liners formed of a rigid or semi-rigid material, which liners are fabricated in sections. For example, the liners are fabricated as four separate quadrants. Each quadrant comprises a curved molded member which may, for example, be thermo-formed. Each molded member is provided with a plurality of projections each having a ducktail cross-section for securing the liner into the concrete structure. The liner sections are joined together and caulked along their engaging edges. The projections of ducktail cross-section extend outwardly from the convex surfaces of the mold members which are arranged with an interior mold assembly and are either joined against the surface of an interior mold member by standard plastic banding or are alternatively joined together end-to-end by individual holding members. These assemblies have been found to lack suitable structural strength and present additional problems in their handling and assembly. In addition, the caulking material has been found to provide unsatisfactory waterproof seams within the manhole assembly structure.